Perforation corrosion of hem flanges in automobiles and trucks, including door, hood, and decklid hem flanges, is the major source of vehicle corrosion warranty repairs. A hem flange is a technique for joining and sealing edges of sheet metal structures involving at least two sheets of material in which one sheet (the outer sheet) is folded or hemmed to accept the second sheet (the inner sheet). These repairs cost manufacturers millions of dollars every year.
As a result, manufacturers have been working to improve hem flange corrosion performance for many years. The current practice in corrosion engineering is to build test modules or vehicles with the desired attributes, and then run the modules or vehicles through a corrosion chamber test or through a proving ground test. Perforation corrosion is measured by visual inspection, or by physically cutting the hem flanges open and examining the extent of corrosion after a certain period of time in the field, in the proving ground, or in the corrosion chamber. At best, these studies provide a qualitative comparison among different materials and systems. The test methods measure corrosion after it occurs, rather than predicting whether it will occur. These test procedures take a long time (about 10 to 12 months) and are very expensive (approximately $100,000 per vehicle).
It is well known that manufacturing process variations contribute to the poor corrosion performance of many vehicles. Among the process variables believed to contribute to corrosion performance are variation in hem flange opening, the precision of the structure adhesive application, and E-coat (electrolytic or electrophoretic coating) bake temperature. However, the high cost of performing the current corrosion tests prohibits running tests with a statistically significant sample size in order to collect enough data to quantify and evaluate the significance of each process variable using the present methods.
U.S. Pat. No. 5,859,537 discloses a non-destructive method for evaluating corrosion on painted metal surfaces. However, the sensor used is relatively small, approximately 1 cm2 in contact with the coated metal surface. Thus, the results only reflect conditions in close proximity to the sensor. It will not allow evaluation of the entire length of the hem flange.
Therefore, there is a need for a simple, non-destructive method of predicting perforation corrosion performance. The test should be quick and inexpensive, and it should also allow evaluation of the entire hem flange.
These needs are met by the present invention which is a method for predicting perforation corrosion in a hem flange. The method includes placing an electrode in the hem flange. The electrode extends substantially the entire length of the hem flange. An electrolyte is introduced into the hem flange so that the electrode is immersed, and the electrochemical impedance spectrum of the hem flange is measured. The measured electrochemical impedance spectrum of the hem flange can be compared to a reference electrochemical impedance spectrum. A collection of reference electrochemical impedance spectra can be created by storing the measured electrochemical impedance spectrum of the hem flange.
Accordingly, it is an object of the present invention to provide a fast, inexpensive, non-destructive method for predicting perforation corrosion along the length of a hem flange.